Proper Definition of Spin Current in Spin-Orbit Coupled Systems

Transcription

1 Proper Definition of Spin Current in Spin-Orbit Coupled Systems Junren Shi ddd Institute of Physics Chinese Academy of Sciences March 25, 2006, Sanya Collaborators: Ping Zhang (dd) Di Xiao, Qian Niu(UT-Austin ) Phys. Rev. Lett. 96, (2006)

2 Outline Introduction New definition Conclusion 1 Introduction 2 Proper definition of spin current 3 Conclusion

3 Concept of Spin Current Key concept Spin current: Spin transport Spin-based information exchange More general than the spin polarized current I I Intuitive definition of spin current: I s = I I Pure spin current: I = I I c = I + I = 0 I s = I I = 2I

4 Generating Spin Current Spin Hall Effect Generating pure spin current by applying electric field Present in non-magnetic semiconductors Mechanisms: Extrinsic mechanism spin dependent skew scattering [Dyakonov and Perel 1972; Hirsch 1999; S. Zhang 2000] Intrinsic mechanism spin dependent anomalous velocity (Berry phase in momentum space) [Murakami et al. 2003; Sinova et al and many others]

5 Spin Accumulation Experiments Y.K. Kato, R.C. Myers, A.C. Gossard, D.D. Awschalom, Science, 306, 1910 (2004). Experiments boundary spin accumulation Theory bulk spin current J. Wunderlich, B. Kaestner, J. Sinova, T. Jungwirth, Phys. Rev. Lett. 94, (2005). Determine the spin current from the spin accumulation?

6 Spin current and spin accumulation Simplest theory: S t + J s = S τ s Sdn = J s nτ s However, spin current/spin accumulation relation is nontrivial: The relation is valid only for the specific form of spin relaxation. Boundary contribution bulk spin current is not the only source contributing to the boundary spin accumulation. Spin accumulation may not be an appropriate way to determine the spin current.

7 Spin Accumulation: Boundary or Bulk Contribution Origins of spin accumulation: Boundary effect: Boundary induced spin density wave similar to Friedel oscillation for charge density. Boundary spin torque: scattering by boundary may induce spin flipping. Bulk contribution: spins are transported to the boundary region from the bulk spin current. J s λ F /2 S z (r) S z (r) l D Spin current is only relevant to the spin accumulation contributed by the bulk.

8 Can the Spin Current Really Describe the Spin Transport? Issues: Electron is localized along x-direction by the impurity scattering. Spin current is non-zero due to the spin-flip scattering The electron cannot contribute to the boundary spin accumulation even it carries nonzero spin current. The spin current is NOT continuous. The spin current does not vanish even in a localized state. The spin current cannot describe the spin transport when spin is not conserved!

9 Fundamentally Flawed Definition of Spin Current Conventional definition of spin current: J e = eυ J s = ŝ z υ However, this definition is fundamentally flawed: Not conserved in spin-orbit coupled systems S z t + J s = T z 0 Not vanishing even in localized states Rashba, 2003 No conjugate force exists Not a standard flow in the sense of the non-equilibrium statistical physics. Current Conjugate force dq/dt J e E j e E J s??

10 Motivation The conventional spin current: cannot be directly measured by any known procedure; cannot descibe true spin transport. The proper definition of spin current must be: It must: describing the true spin transport. measurable as a macroscopic current. conserve: S z t + J s = 0 vanish in (Anderson) insulators be in conjugation to a force spin force

11 A Conserved Spin Current Continuity equation Assume zero spin generation in the bulk Torque dipole density S z t + J s = T z ṡ z 1 dv T z (r) = 0 V T z (r) = P τ (r) Current conserved S z t + (J s + P τ ) = 0 New definition: dv P τ = J s = J s + P τ dv ṡ z r d(sz r) J s = s z ṙ + ṡ z r = dt

12 Spin Torque Dipole Definition of Spin Torque Dipole T z (r) = P τ (r) Macroscopic average: 1 dv P τ (r) = 1 V V dv rt (r) Analogy to the charge dipole density: P(R) = 1 rρ(r) V V R

13 Effective Conserved Spin Current Operator dv P τ (r) dv J s (r) dv J s (r) = dv rt (r) dv Reψ (r) 1 { 2 dv Reψ (r) 1 { } dˆr 2 dt,ŝ z ψ(r) [ ] dv Reψ d(ˆrŝz ) (r) ψ(r) dt ˆr, dŝ z dt } ψ(r) Ĵ s = d(ˆrŝ z) dt This is not microscopic definition of the spin current operator it is an effective one defined in the macroscopic level.

14 Testing Case T T T T Spin torque density T z is non-zero spin flip process. Spin torque dipole: P τ = T l Conserved spin current: J s = J s + P τ = 0 Spin torque dipole deducts the local polarization contribution from the spin current. The resulting conserved spin current describes the true spin transport.

15 In Insulators Zero expectation value in any spatially localized states: l Ĵ s l l d(ˆrŝ z) l dt = E l E l l ˆrŝ z l = 0 i Zero spin transport coefficient in Anderson insulators: σ s = e l l f l Im l d(ˆrŝ z )/dt l l ˆυ l (ǫ l ǫ l ) 2 = e l f l l [ˆrŝ z, ˆr] l = 0

16 Conjugate Force for spin current Origin of spin force: gradient of an inhomogeneous Zeeman field spin dependent chemical potential near ferromagnet-metal interface dq dt dh 0 dt H = H 0 ŝ z ˆr F s = d(ŝ zˆr) dt F s J s F s The new definition conforms to the standard near-equilibrium transport theory.

17 Onsager Relation for Charge/Spin Transport Spin Hall effect Inverse spin Hall effect j s E j e F s J x s = σ sc xye y j y e = σ cs yxf x s Onsager Relation: σ sc xy = σ cs yx The spin transport can be connected to the charge transport.

18 Onsager Relation General theory A system under two driving forces: H = H 0 X 1 F 1 X 2 F 2 Transport coefficients defined by: Ẋ1 = σ 11 F 1 + σ 12 F 2 Ẋ2 = σ 21 F 1 + σ 22 F 2 Onsager relation σ 12 (S) = s 1 s 2 σ 21 (S ) Direct application: X 1 eˆr X 2 ŝ z ˆr

19 Direct measurement of spin current Thermodynamic method: J s = 1 F s dq dt Technique to measure the Zeeman field gradient is required. Electric method: J s = σ sc V y xy L y σ sc xy can be determined from the inverse spin Hall effect: σ sc xy = σ cs yx V y σ ss F s Ey dq/dt J s J s

20 Linear Response Theory To calculate the spin torque dipole: Calculate the spin torque response to external field at finite wave vector q: T (q) = χ ν (q)e ν (q) Spin torque dipole is related to the spin torque by: Long wave limit q 0: T (q) = iq P τ (q) iq µ σ τ µνe ν (q) σµν τ 1 = i lim χ ν (q) = i µ χ ν (q) q 0 q q 0 µ

21 Intrinsic Spin Hall Coefficients Conventional values: Rashba model: e 8π Cubed-k Rashba model: 9e 8π Luttinger model: New values: Rashba model: e 8π Cubed-k Rashba model: 9e 8π Luttinger model: 3eγ 1 12π 2 γ 2 (k H k L )+ e 6π 2k H γ 2 0: e 6π 2k e(γ 2 γ 1 ) 6π 2 (k H k L )+ e γ 2 6π 2k H γ 2 0: e 6π 2k

22 Disorder effect (a) Rashba model Impurity potential Born approx. Definition of spin current J s J s δ(r) 1st 0 0 higher 0 0 V p p 1st 0 0 higher 0 Finite (b) Cubic Rashba model Impurity potential Born approx. Definition of spin current J s J s δ(r) 1st Finite 0 higher Finite 0 V p p 1st Finite 0 higher Finite Finite Sugimoto, Onoda, Murakami and Nagaosa, cond-mat/

23 Conclusion A proper definition of spin current is established: Conserved Kirchhoff s law for spin current Vanishes in Anderson insulators True transport current Measurable Conjugate force exists d(sz r) J s = s z ṙ + ṡ z r = dt Physical consequences: (somewhat disappointing) A few widely studied semiconductor models (Rashba and cubic Rashba) turn out to have NO intrinsic spin Hall effect. There is still NO known non-magnetic system that can generate a spin current in the presence of an electric field.

24 Thank You For Your Attention!